Home Improvement Asked on March 17, 2021
I originally planned to install a subpanel and feed it 90 amps. I thought I could do this by putting a 90A double-pole breaker in my main panel and feeding off that. What I was told was "This will supply 90A on each leg, so your shop will really have 180A", but I don’t think that is right. Does that truly supply 90 amps on each leg or does this get reduced to 45A on each leg (i.e. if I’m running equipment at 240 V then I have 90A available, but all the 120V stuff is limited to 45A)?
I think where this could bite me is if I’m running a welder at 240V and 50A (25A per leg) and somebody else is running the table saw and the compressor kicks on (assuming saw and compressor are on the same leg). This scenario will trip the breaker, correct?
Common misunderstanding. If you supply your sub-panel with 90 amps at 240 volts, that's what it will have. You'll still have 90 amps available on 2 legs at 120 volts. So yes, you'll have 180 amps available at 120 volts (90 amps on each hot (leg)).
If possible, can you run your table saw and compressor at 240? Many motors can easily be converted from 120 to 240 with a simple wiring change. See the face plate on the motor for instructions. If not, be sure they are on separate 120 v legs. And just because the instructions for a table saw says it needs a certain amperage, the reality is tools like a table saw rarely draw their full load. I know others here tend to over-kill the supply, but I'm a bit more realistic to real-world considerations.
Correct answer by George Anderson on March 17, 2021
240V @ 90A = 21.6 kW
Or 120V at 90A (twice) = 180A = 21.6 kW
If your compressor and tablesaw are 120V, consider changing them to 240V, unless you also haul them off to jobsites that don't have 240V available.
If you are running a 240V welder at 50A, that's 50A per leg (& nothing on neutral.) 240V uses both legs.
You would then have 40A left on your 90A breaker, whether at 120V or 240V.
If the tablesaw and compressor are both 20A 120V and both on the same leg of the service, it might be a near thing with startup surges - but technically it is within 90A total: 50 on one leg, 90 on the other leg, 40 on neutral.
If they are 20A 120V and on opposite legs, no problem. 70A total, nothing on neutral.
If they are 10A 240V, also no problem. 70A total, nothing on neutral.
Answered by Ecnerwal on March 17, 2021
No no, it doesn't work that way! Loads don't divide!
There is no such thing as a 30A-nameplate load drawing 15A on two legs. That kind of "division" does not happen anywhere.
If you have a 50A EVSE, that is pulling 50A from leg L1 and 50A from leg L2.
If you have a 12A heater, that is pulling 12A from leg L1 and 0A from leg L2.
You might think, "Well, that's a lot like pulling 6A from both legs". If you think that, you're starting to get in the ballpark, but still... that balancing isn't going to happen without a transformer being involved.
Answered by Harper - Reinstate Monica on March 17, 2021
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